ABSTRACT
Transgenic milk production offers a cost-effective system for the manufacturing of large amounts of complex proteins. Specifically,
commercialization is near for recombinant human antithrombin (rhAT) expressed in transgenic dairy goats. The product received
a positive opinion from the Committee for Medicinal Products for Human Use of the European Medicine Agency.
This article reviews the reasons why transgenic milk is a cost-effective system. Also reviewed is the earlier research on
targeting heterologous proteins to the mammary glands of many different animals. The final section describes the process by
which goats express rhAT in their milk at approximately 2 g/L. The human AT purified from milk is structurally indistinguishable
from human plasma–derived AT with the exception of carbohydrates. Clinical studies are ongoing on the prevention of deep-vein
thrombosis.
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The use of recombinant proteins has steadily increased during the last two decades. A large number of human proteins and potential
therapeutic targets and their development for therapeutic uses have been identified. Clinical applications often require large
amounts of highly purified molecules, for multiple or chronic treatments. The development of very efficient expression systems
has been the key to the full exploitation of the recombinant technology. Thanks to a careful integration of molecular biology,
large animal embryology and protein chemistry, transgenic milk production offers a cost-effective system for the manufacturing
of large amounts of complex proteins.
Recombinant human antithrombin (rhAT, commercial name ATryn) is the most advanced of the transgenic milk-derived compounds.
After several years in clinical development, it has recently received a positive opinion from the CHMP (Committee for Medicinal
Products for Human Use) of the EMEA (European Medicine Agency) for its market application authorization for the prophylaxis
of venous thromboembolism in surgery of patients with congenital antithrombin deficiency. This was the first positive opinion
by a regulatory agency for a transgenically produced biopharmaceutical (from either plant or animal sources). 
Various risk minimization measures have been instituted to protect this highly controlled closed donor goat population.
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This article traces the development of protein technology from the microbe to the mammary glands. It will close with specific
information on rhAT production and the status of clinical tests.
LOW- VOLUME, NON-MILK TRANSGENIC PROTEINS
The first microbial bioreactors, in particular Escherichia coli and Saccharomyces cerevisiae, were found to be satisfactory for the production of simple polypeptides such as insulin and human growth hormone. However,
microbial bioreactors were found to be unsuitable for proteins with complex post-translational modifications or intricate
folding requirements, such as the coagulation factors, or monoclonal antibodies. This led to the development of large-scale
mammalian cell culture, for example, the use of Chinese Hamster Ovary (CHO) cell bioreactors.
These technologies permitted the development of numerous monoclonal antibodies, cytokines, and other complex bioactive biomolecules.
However, there are proteins that, due to a combination of complex structure and large therapeutic dosing, have until now eluded
recombinant production using traditional bacterial and cell culture bioreactors. For example, commercial recombinant production
of complex molecules, such as antithrombin and alpha1-antitrypsin, has not yet been achieved in microbial or mammalian cell
derived bioreactors. The only source is human plasma because of the high dose needed.
Even human serum albumin, the therapeutic protein used in largest amounts (>400 tons, worldwide), the use of the recombinant
form, produced in Saccharomyces cerevisiae, is limited to excipient applications. These are within the practical production capacity of this system but far too small
for high-volume therapeutic indication (volume replacement).
Capital investments in production plants represent a significant portion of the development cost of new recombinant drugs.
Also, the inherent risk associated with the regulatory approval process is a stimulus for the development of flexible and
inexpensive approaches for the manufacture of therapeutic proteins. Milk-specific production offers a way to lessen the bite.
